In paper mills, various paper and board grades are produced in a paper machine by draining water out of a mixture of water and wood fibers prepared in a pulp preparation plant. Water is an essential raw material in the production of paper. Besides operating as a binder and carrier agent for the fibrous material which is called a paper web, water is also needed, among other things, for washing, sealing, lubrication, carrying away disturbing materials (contaminants), and for transferring of thermal energy. In different parts of a paper machine, constantly moving fabrics are used, and for example, such fabrics are present in the wire part, in the press section, and in the dryer section. The fabrics in a paper machine are permeable members which form a closed loop (i.e., are endless) and which are typically made of a plastic and/or a metal material or the fabrics may be felts consisting of natural and/or synthetic fibers. The fabric loops are rotated constantly by means of drive rolls or by means of some other equipment. While draining water from the paper web, the fabrics are contaminated by materials which come from the paper web and from the different process waters. In order that the fabrics and the elements in the paper machine, such as rolls, doctors, forming ribs, suction boxes, etc., should operate well, they must be washed constantly by means of water jets, and the wash water must be removed. The wash water from the fabrics is contaminated, but it can, however, be used as circulation water in the paper machine.
In present-day paper mills, an abundance of fresh water is needed for cooling and, after that, among other things, for the above washing requirements in the wire part and in the press section and for dilution in the stock preparation plant. After the wire part and the press section, these waters are passed mainly to mix with fibrous circulation waters. Any excess amount of circulation water is disposed of as waste water. The net amount of fresh water that is needed for the washing jets in a paper machine is of an order of about 10 cubic meters per ton of paper product produced. Thus, from a paper mill, an abundance of warm waste water is obtained, which must be cleaned, for example biologically, and, if necessary, cooled before the cleaning.
Materials that interfere with the operation of the process enter into the process mainly from the stock preparation and from the broke circulation in the paper machine. Unless the process is connected with separate cleaning devices that remove the disturbing materials, the disturbing materials have just two possible routes of removal: either along with the product or along with the waste waters.
Since the amount of disturbing materials produced in the papermaking process per product ton depends mainly on the raw material used and on the product quality, the concentration of the disturbing materials in the water circulations in the process is relatively linearly dependent on the amount of fresh water that is used.
As known in the prior art, the wash jet waters or water flows in a paper machine are collected by means of various basins and troughs and passed into a circulation water system. Besides fresh water, circulation water of the paper machine is also employed as the jet water in the wire part and the press section. The circulation water is usually cleaned by means of filters, whose screen measure is about 150 .mu. (corresponding to about 100 mesh). Such a screen measure, however, permits the passage of fine particles and dissolved material. A clear filtrate obtained from such a filtering device still contains finer particles and dissolved material. If this clear filtrate is used for jet nozzles, these impurities may cause blocking of the jet nozzles and their structures with the disturbing materials and contamination of these devices and other equipment in the water system in a paper machine, which results in negative effects in the quality and production of paper. The use of such water as additional substitute for fresh water would risk impairing the operation of the equipment and the production of the paper. Therefore, for more demanding washing of the fabrics and parts of a paper machine, fresh water is used, which is mostly chemically cleaned prior to actual use.
Fresh water is cold, and it must be heated to a considerable extent to the operating temperature that is required in the paper-making process. The temperature of new fresh cold water must be raised, for example, from about 7.degree. C. to about 50.degree. C., and usually it is treated chemically in order to remove humus materials and color, in compliance with the quality requirements, and its use involves high costs of processing. The high cost of cleaning of fresh water and waste water arises from the fact that an abundance of fresh water must be introduced into the process constantly. Fresh water which is used in the jets in a paper machine and which has been treated chemically also increases the concentrations of inorganic materials in the system.